I know for a fact he revised every day for Maths for weeks, we bought a revision guide, I asked his teacher for the topics in plenty of time. We set him work ourselves. This was not a lack of revision. He put in far more preperation for Maths than any other subject.
His Maths teacher says he's perfectly capable and grasps the concepts.
Ok, you want to know what is going wrong but you haven't said anything about how he is actually getting on with the maths he's doing.
His maths teacher says he is capable in lessons, grasps the concepts. You say he has revised lots so it's not lack of work. You also say that he needs support with homework and also that he has done badly in maths exams since primary.
So is he capable and grasps the concepts and just does badly in exams (which could be issues with anxiety around timed exams) or is he not actually capable and doesn't grasp the concepts?
When he was doing all this revision, was he actually getting the work right? You said you were setting him work, were you marking it? What sorts of mistakes was he making? Was he able to eventually get questions right? Was he then able to get the same questions right the next day? It is quite easy in maths to do lots of 'revision' and not make the slightest bit of progress through doing the wrong things (e.g. just copying examples, or just watching videos or doing a lot of questions and not marking them so not being aware they're wrong, or getting them wrong then not doing anything about it).
One thing to check is the 22% mark - how does this compare to the rest of the class? Is the teacher concerned? Because at GCSE on a foundation paper, 22% could be a grade 4 pass and be fine, it does depend on the level that the paper was set at and the expectation of the group he's in.
There are lots of questions that need answering before a plan of action can be implemented - a priority would be to get the exam paper back and have a look at where he is going wrong. Were there questions that he definitely should have got right, that you know he knew how to do, that he didn't?
I don't know about what you can expect from a private school, but asking the teacher to look through it with him and reporting back what the teacher thinks the issues are would be helpful.
(Dyscalculia, by the way, unlike dyslexia, isn't really a recognised thing. Generally children who display signs of 'dyscalculia' either have other SEN like poor working memory that particularly comes to the fore in maths, or poor mathematical foundations that create problems with later learning).